Weak-form Efficiency
Also written Weak-form EMH · Weak form of efficiency · Weak-form efficient market hypothesis
The hypothesis that current prices already reflect all historical market information — past prices, returns and volumes — so technical analysis cannot earn above-average risk-adjusted returns.
In plain language
Eugene Fama split the efficient market hypothesis into three levels, according to what information prices are assumed to reflect. Weak form is the first and least demanding.
It says today's price already contains everything that can be learnt from the market's own history: the sequence of past prices, rates of return, trading volume.
If that is true, current prices are independent of past prices. Price changes follow a random walk. And a chart pattern, a moving average or a momentum signal — anything built only on past price and volume data — cannot reliably tell you where the price goes next.
How it works
The three forms nest inside each other:
| Form | Prices reflect | Rules out superior returns from |
|---|---|---|
| Weak | All historical market data — prices, returns, volume | Technical analysis |
| Semi-strong | Historical data + all public information | Technical and fundamental analysis |
| Strong | Historical + public + insider information | Any information, for any group |
The workbook's implication for technical analysis is blunt: if the market is weak-form efficient, analysing past information will not identify future price movements, "as past returns are not congruent with future returns". EMH "renders technical analysis completely irrelevant for superior returns". One should gain little from any trading rule based on past rates of return or other past market data.
The early academic treatment was the Random Walk hypothesis: successive one-period returns are independent and identically distributed. Fama formalised the theory in 1970.
A worked example
Illustrative figures. A trader notices that a mid-cap pharma stock at ₹1,480 has just crossed above its 200-day moving average on rising volume — a classic buy signal. She buys 500 shares, about ₹7.4 lakh.
Under weak-form efficiency, the signal carries no edge. Every other participant watching the same chart saw the same crossover at the same moment. If the pattern reliably preceded a rise, buyers would already have bid the price up to where the expected risk-adjusted gain vanished.
Suppose over a year the trader runs 40 such signals and the stock-level results average out to the market's return. After brokerage and taxes on 80 buy and sell orders, her net return trails a simple index position of the same ₹7.4 lakh. That is exactly the weak-form prediction: no abnormal return from past price data, and costs as a guaranteed drag.
What weak-form efficiency does not say: that the trader cannot make money. She can, if the market rises. It says she cannot consistently beat a comparable-risk passive position using charts alone.
Why NISM asks about it
Chapter 13 (Concept of Informational Efficiency), sections 13.2.1 and 13.5.1, defines weak-form efficiency and draws its implication for technical analysis. A Chapter 13 sample question asks what the weak form states — successive price changes are independent. Chapter 18 returns to the tension when it describes momentum investing, which relies on exactly the price-volume data weak-form efficiency says is already priced.
Common exam traps
- Weak form rules out technical analysis, not fundamental analysis. Fundamental analysis is ruled out only under semi-strong form.
- Successive price changes are independent — the correct statement. "Dependent" or "biased" is the wrong option.
- The forms are cumulative. A semi-strong-efficient market is also weak-form efficient; not the other way round.
- Efficiency does not mean price = true value at every moment. The workbook's technical definition: price is an unbiased estimate of value, with random deviations uncorrelated with any observable variable.
- "Above-average risk-adjusted returns", not "any returns". Efficient markets still pay normal returns for risk.
Where this is taught
Free preparation for NISM Series XXI-BRelated terms
- Efficient Market HypothesisThe proposition that share prices already incorporate and reflect all relevant information — which, if true, leaves nothing for an analyst to find by studying that information.
- Technical analysisForecasting price direction from past price and volume alone, on the assumption that everything worth knowing about a company is already in its price.
- Market anomalyA pattern of returns that departs from what an asset's fundamentals and risk — as measured by CAPM or another pricing model — say it should earn, and so contradicts market efficiency.
- Semi-strong-form EfficiencyThe hypothesis that prices reflect all historical and publicly available information and adjust rapidly to news — so neither technical nor fundamental analysis earns above-average risk-adjusted returns.
- Strong-form EfficiencyThe most extreme form of the efficient market hypothesis: prices reflect all historical, public and insider information, so no group of investors can earn above-average risk-adjusted returns.
- Informational EfficiencyHow fully and quickly security prices reflect available information. It is what "market efficiency" usually means, as against operational efficiency, which is about transaction costs.
- Random Walk HypothesisThe early form of the efficient market model: security price changes occur randomly, so successive one-period returns are independent and identically distributed.
- Fair Game ModelEugene Fama's 1970 framing of the efficient market hypothesis: current price fully reflects available information, so no investment strategy can consistently earn above-average risk-adjusted returns.
- Operational EfficiencyHow cheap it is to transact in a market — the cost of carrying out trades — as distinct from informational efficiency, which asks whether prices already reflect all available information.
- Wash SaleA US tax rule, cited by the workbook to explain the January Effect: it disallows a loss deduction when an investor sells a security at a loss and buys back a substantially identical one within 30 days before or after.
- January effectA market anomaly in the US: year-end tax selling pushes prices down in late November and December, and the buying back in early January pushes them up — a pattern inconsistent with market efficiency.
- Internal contradiction of efficiencyThe paradox that a market can only become informationally efficient if enough participants believe it is not — because it is their search for mispricing that drives prices to fair value.
- Unbiased EstimateThe technical definition of an efficient market price: the market price is an unbiased estimate of a security's true value, so deviations are random rather than one-sided.